Ordered hints in Oracle:

Question

There's a fairly popular Ordered Hint example on the web as follows:
<CODE>
select /*+ ordered use_nl(bonus) parallel(e, 4) */
e.ename,
hiredate,
b.comm
from
emp e,
bonus b
where
e.ename = b.ename
</CODE>
I would like to know what the "parallel(e, 4)" clause does. Where does the "4" come from? What is "parallel" here?
I also have another question: If I have 5 tables--T_OREGON, T_UTAH, T_VIRGINIA, T_TEXAS, and T_OKLAHOMA--lined up in a join right behind a FROM, coming in at row counts of
T_OREGON: [a lot of rows; a lot more than T_UTAH]
T_UTAH: [smaller than T_OREGON] 40550 rows
T_VIRGINIA: 14 rows
T_TEXAS: 66 rows
T_OKLAHOMA: 8 rows
like so:
...from T_OREGON or, T_UTAH ut, T_VIRGINIA va, T_TEXAS tx, T_OKLAHOMA ok...
my question is, if an Ordered hint can be used here, is it that the smallest table (in this case T_OKLAHOMA) gets placed first in the join and the rest of the table ascension in the join doesn't matter? Or is it that T_OKLAHOMA gets placed first in the join, followed by T_VIRGINIA [at 14 rows], followed by T_TEXAS, T_UTAH, and finally T_OREGON?
Thank you for any help that you can provide.

Answer 1

It is why my other question involved the table order
behind a FROM keyword, (when employing an Ordered
hint).
Can anybody help with that question? Thanks again.With rule based optimizer order was important.
With cost based optimizer (optimizer mode one of the all_rows, first_rows, forst_rows_n) order is not important Oracle itself chooses the order how to join tables.
If you have gathered statistics leave CBO do his work. See more about optimizer here http://download.oracle.com/docs/cd/B19306_01/server.102/b14211/optimops.htm
And if you have taken this citation out of http://articles.techrepublic.com.com/5100-22_11-1050199.html then there are at least following problems:
1) article is 4 years old
2) article doesn't say anything about downsides using this hint e.g. what if your data changes and order is not relevant anymore, quite high possibility that query is changed but hint forgotten to change, maintaining additional level of complexity where Oracle has invested many many resources to assist users enhancing CBO
3) and there is at least 1 fallacy in this article these sentences are wrong:
"When Oracle evaluates table join orders, it must consider every possible combination of tables. For example, a six-way table join has 720 (permutations of 6, or 6 * 5 * 4 * 3 * 2 * 1 = 720) possible ways that the tables can be joined together. This permutation issue becomes even more pronounced when you have more than 10 joined tables in a query: For a 15-way table join, there are over one trillion (1,307,674,368,000 to be exact) possible query permutations that must be evaluated."
(emphasis is mine - Gints)
CBO abandons each permutation as soon as the cost is higher than the smallest from previous steps. So in reality CBO is much smarter than article tries to explain.
If you really want to know how CBO works then I can only suggest to get book Cost based oracle fundamentals by Jonathan Lewis.
Gints Plivna
http://www.gplivna.eu